We are studying levels of proinsulin in the blood of normal subjects, pregnant women and obese and diabetic patients. The concentrations of proinsulin and insulin are measured both in the fasting state and after the administration of various stimuli such as glucose, tolbutamide and glucagon. In addition, with the development of an antiserum to the human C-peptide we are measuring this peptide in blood after similar stimuli. As the C-peptide and insulin are secreted in equimolar amounts we hope that its level will prove to be a useful indicator of beta cell activity in conditions of pancreatic dysfunction and in insulin requiring diabetic subjects. Investigations are underway to determine the interfering effect of various concentrations of proinsulin on the conventional insulin radioimmunoassay procedure. As different insulin antisera cross-react to a variable extent with proinsulin, we will check its effect with a number of selected antisera. The results of these experiments will be compared with sera which contain low and high amounts of endogenous proinsulin relative to insulin. The peripheral metabolism of proinsulin will be studied in animals with regard to its half life, tissue distribution, organ extraction and urinary excretion. The possibility of the peripheral conversion of proinsulin to insulin will be investigated. The biological activity of insulin and proinsulin are being compared in a number of in vitro systems, including rat diaphragm muscle and epidydymal fat tissue, and toad skin.